Most people these days are concerned with some miner pool commanding >51% of the hashing power but my concern is also related to what would happen if for whatever reason a significant portion of the hashing power vanishes. Block confirmation times would increase and eventually difficulty would be reset downwards and things would go back to normal but if the hashing power lost is significant, it could take several weeks or months until this would happen. Isn't there a shortcut in the protocol?
A built in "shortcut" could be exploited by an network attacker who has isolated your node to feed you fake confirmations— so no. If there is some kind of massive coordinated loss we likely have much worse to worry about (like that hashpower actually being off on a separate partition that will overtake our current chain). Of course "small" losses, like "half" are no big deal— 20 minute blocks rather than 10.
Efforts in altcoins to 'fix' behavior here have traded off security in the common case for security in cases where the system is already broken for other reasons— in some cases they've been exploited pretty hard as a result, so I don't consider it a good tradeoff at all.
